1. Field of the Invention
The present invention relates to a laser source and, more particularly, to a laser source suitable for an integrated optical element such as a spectrum analyzer.
2. Related Background Art
A spectrum analyzer has been proposed as an application of an optical integrated circuit. The principle of operation of the spectrum analyzer will be described with reference to FIG. 1. A beam emitted from a semiconductor laser 103 coupled to an end face of a slab waveguide path 101 propagates as a divergent beam 108 along the slab waveguide path 101 and is collimated by a waveguide path lens 104. A collimated beam wave 109 is diffracted by an elastic surface wave 112 excited by an interdigital electrode 106 and is then Fourier-transformed into beam waves 110 and 111 by a waveguide path lens 105. In this manner, the Fourier transformation surface is set to be an end face, and a line sensor such as a CCD sensor is arranged at the end face, so that spectrum analysis of an applied RF (Radio Frequency) signal can be performed. In addition to the above application example, when semiconductor lasers are used in optical integrated circuits in a variety of applications, any beam collimating means such as a waveguide path lens is required in a slab waveguide path. Therefore, fabrication of devices is time-consuming and cumbersome. In addition, it is difficult to produce compact devices which are used in practice.
A linear light source which does not require a collimating means is described as a laser in "Edge-and surface-emitting distributed Bragg reflector laser with multiquantum well active/passive waveguides", K. Kojima et al., Appl. Phys. Lett. 50, 5, PP. 227-229(1987). This laser is a DFB (Distribution FeedBack) laser which utilizes second-order diffraction as reflection. Part of the optical output is coupled to a first-order diffracted wave and is output in a direction perpendicular to a substrate.
In this example, however, since beam waves bidirectionally propagate in a resonator, two beams having a very small output angle difference are output. When these beams are focused, two beam spots are undesirably formed. In addition, the output direction is perpendicular to the surface of the substrate. When this substrate is coupled to another waveguide path, i.e., another substrate, these substrates must be located adjacent to each other. If these substrates include an LD (Laser Diode) which emits light from the upper surface having electrodes, the composite structure is complicated.
The present inventor proposed structures in Japanese Patent Laid-Open (Kokai) Nos. 62-123411 and 62-124510 (corresponding to U.S. Pat. No. 4,776,661). In these structures, a beam diffracted from a channel waveguide path 124 having a grating 125 shown in FIG. 2 is used as a means for obtaining a collimated slab waveguide beam. A beam wave 126 output from a semiconductor laser 123 coupled to an end face of the channel waveguide path 124 is Bragg-diffracted by the grating 125 having a uniform period. Part of the beam wave 126 is coupled to a slab waveguide beam 127 which propagates along the slab waveguide path. By using the grating having the uniform period, in-phase collimated beams can be produced.